A method for sensing a register discrepancy in a multi-color printed item, the position of at least one register mark that is associated with one color being sensed with respect to the position of at least one register mark that is associated with at least one further color, and any register discrepancy being ascertained from discrepancies in the actual relative position with respect to the reference relative position.
The invention furthermore concerns an apparatus for carrying out such a method, a register control system having such an apparatus, and register marks for carrying out the method.
The purpose of sensing register discrepancies is to control multi-color printing presses, since the individual single-color images must be exactly superimposed on one another. This exact alignment is brought about by the fact that first register marks are printed with the first printed single-color image, and further register marks are printed along with the subsequent single-color images. By way of an optical scanning apparatus, the positions of the register marks printed first (viewed in the transport direction of the paper) are compared to the register marks printed subsequently. If a positional displacement occurs in the register marks that are to be compared, the phase position of the individual printing rollers or of their printed images is adjusted so that the register marks, and thus also the individual single-color images, are aligned relative to one another within the desired tolerance range.
A method of the kind cited initially, an apparatus for carrying it out, a register control system, and register marks for such a method are known from DE Unexamined Application No. 27 31 914. In this, a series of test marks with gaps are provided; some of them serve to identify the register sensing region within the overall printed item, and a gap between two test marks represents the actual register mark for sensing register. This requires, however, separate optical scanning apparatuses configured specially for sensing the register marks and their exact position, thus marking the presses more expensive and requiring valuable additional space.
It is, therefore, an object of the invention to eliminate the optical scanning apparatuses which serve exclusively for the sensing of register marks.
With regard to this method, the object is achieved in that the possible register discrepancy between colors, for which the sum of adjacent individual densities is not equal to the total density when the colors are superimposed, is ascertained by the fact that first the individual densities of the colors and their total density when printed onto one another is measured; then the density of the register marks is measured in a measurement window; and then, from the measured density values, the degree of overlap of the register marks in the measurement window, and from that the register discrepancy, is measured.
In terms of the apparatus for carrying out such a method, the object is achieved in that it has at least one densitometer and one analysis device, the at least one densitometer being provided for the sensing of individual densities of colors and of a total density of at least two colors printed onto one another, and for measuring the density of the register marks in at least one measurement window; and the analysis device being configured such that from the measured density values, it ascertains the degree of overlap of the register marks in the at least one measurement window and, from that, the register discrepancy.
There is proposed for the apparatus according to the present invention a register control system in which there is connected to the aforesaid analysis device a control device which is configured such that, in accordance with the register discrepancies, it issues positioning commands to positioning elements of the individual printing units in order to bring about the reference relative position of the various colors with respect to a reference color.
To carry out the method according to the present invention, register marks are proposed which are configured in planar fashion with defined boundaries for sensing of the density using a densitometer, the sum of adjacently located individual densities of various colors being unequal to the total density when the colors are superimposed, and the arrangement of the register marks being such that the total densities of register marks within a measurement window can be sensed by a densitometer.